1) Field of the Invention
The present invention relates to an apparatus for measuring fluorescence lifetime based on number of measured photons generated from a sample by irradiating an excitation light on the sample.
2) Description of the Related Art
Recently, researches on methods of calculating fluorescence lifetime of various samples are so popular that lots of methods have been proposed. By irradiating an excitation light on a sample, the molecular state of the sample becomes excited, and a fluorescence is generated while the sample returns to a ground state. The method measures the fluorescence and calculates the fluorescence lifetime to investigate type or state of the sample. One of the methods employs a pulse excitation light to irradiate the sample, and measures the number of photons emitted from the sample in plural time gates (hereinafter, “time gate method). The fluorescence lifetime is calculated based on the number of photons measured in the plural time gates. The time gate method can offer a fluorescence lifetime with a small error if time gates are properly set.
In the time gate method, however, since the probability of emitting photons is generally low, it is required to irradiate the pulse excitation light a number of times. Consequently, it takes an extremely long time to measure the number of photons with reliability. On the other hand, if one increases the probability of emitting photons and sets the time gates to time windows where the probability of emitting photons is high to shorten the time for the measurement, a measurement error becomes large because it is difficult to discriminate each photon when a plurality of photons are incident in the same time gate. For this reason, it is required to measure the number of photons at a very low emission probability where an average of the number of photons being incident in each of the time gates with a single irradiation of pulse excitation light is approximately 0.01 to reduce the measurement error. As a result, for a single calculation of the fluorescence lifetime, at least several ten-thousand-times of irradiation of pulse excitation light is required, and it takes a considerable amount of time to calculate the fluorescence lifetime. In addition, the fluorescence lifetime may change with time even in the same sample. In this case, the time gate method cannot be applied, and a time-dependent fluorescence lifetime change cannot be measured.